Good image quality and minituarization of imaging devices, especially optics, are difficult to combine. Zoom and autofocus functions need space to extend/retract, typically from few millimeters to few tens of millimeters, and the amount and size of high quality lenses need to be fitted inside the structure.
Trendwise the miniaturization of digital still cameras have undergone a transition into very thin, pocket size, devices. From an optics point of view the typical extending & retracting zoom/autofocus features have been formed into smaller shape, folded optics. In these the movement of lenses takes place totally inside the device and a prism is used to fold the viewing angle 90 degrees.
This solution has the drawback that only relatively small lenses can be used for the folded optics since they are placed totally inside the device, but this leads to decreased optical quality of the optical system. Furthermore, the small lenses implicates the use of a relatively small image sensor which also decreases the picture quality.
Unfortunately, the use of bigger lenses in such a folded optic system would lead to a tremendous increase of the device's size.